For anyone interested, the following code solved this problem.
mask = arcpy.GetParameterAsText(3)
MinX, MinY, MaxX, MaxY = mask.split( " ")
MinX = float( MinX)
MinY = float( MinY)
MaxX = float( MaxX)
MaxY = float( MaxY)
The variables were then passed as follows:
outExtent = Extent(MinX, MinY, MaxX, MaxY)
outConstRaster = CreateConstantRaster(constantValue, "INTEGER",
cellSize, outExtent)
(Curtis Price added code formatting)